Assuming the conservative SOTUS judges don’t croak in the next four years, the most likely scenario is that Hillary or Obambi will replace aging liberals. No change to the status quo is likely.
I will not vote for McCocaine.
The only difference is that McCain will nominate liberals like Stevens and Souter and Clinton or Obama will nominate ultra-liberals like Ginsburg and Breyer. There are enough RINOs in the Senate to approve any liberal or ultra-liberal that gets nominated.
It probably will not affect the current status quo on the Court (unless, like you said, one of the conservatives unexpectedly croaks in the next 4 years), but whoever gets in will likely be there for the next 30 years.